Solar cells are electric devices that convert sunlight into electricity by the inner photoelectric effect. FIG. 18 is a simplified diagram showing a conventional solar cell 20 formed on a semiconductor substrate 21. Semiconductor substrate 21 is processed using known techniques to include an n-type doped upper region 21A and a p-type doped lower region 21B that collectively form a pn-junction near the center of substrate 21. Formed on an upper surface 22 of semiconductor substrate 21 are a series of parallel metal gridlines 25 (shown in end view) that are electrically connected to n-type region 21A. A substantially solid conductive layer 26 is formed on a lower surface 23 of substrate 21, and is electrically connected to p-type region 21B.
An antireflection coating 27 is typically formed over upper surface 22 of substrate 21. Solar cell 20 converts sunlight beams L1 that pass through upper surface 22 into substrate 21 when a photon from beam L1 hits a semiconductor material with an energy greater than the semiconductor band gap, which excites an electron (“−”) in the valence band to the conduction band, allowing the electron and an associated hole (“+”) to flow within substrate 21.
The pn-junction separating n-type region 21A and p-type region 21B serves to prevent recombination of the excited electrons with the holes, thereby generating a potential difference that can be applied to a load by way of gridlines 25 and conductive layer 26, as indicated in FIG. 18.
Screen printing is the predominant conventional method for producing the gridlines 25 that carry current from upper surface 22 of conventional solar cell 20. Conventional screen printing techniques typically produce gridlines having a rectangular cross-section with a width W of approximately 100 μm and a height H of approximately 15 μm, producing an aspect ratio of approximately 0.15. A problem associated with conventional gridlines 25 having this relatively low aspect ratio is that gridlines 25 generate an undesirably large shadowed surface area (i.e., gridlines 25 prevent sunlight from passing through a large area of upper surface 22 into substrate 21, as depicted in FIG. 18 by light beam L2), which reduces the ability of solar cell 20 to generate electricity. However, simply reducing the width of gridlines 25 (i.e., without increasing the gridlines' cross-sectional area by increasing their height dimension) could undesirably limit the current transmitted to the applied load. Forming high aspect ratio gridlines using screen printing techniques would significantly increase production costs because it would require very precise alignment of successive prints with intervening drying steps.
Extrusion printing is an alternative to screen printing, and involves passing an extrusion printhead over the upper surface of the solar cell substrate while extruding a metal ink. Extrusion printing has an advantage over screen printing in that extrusion printing facilitates the efficient production of solar cells having high aspect ratio gridlines. The resulting gridline structures have a cross-sectional shape that very closely follows the cross section of the extrusion printhead nozzle, which is typically rectangular. More recently, co-extrusion printing has been introduced to solar cell production in which a metal ink is co-extruded with and encapsulated by a sacrificial ink. Co-extrusion printing can produce lines of 50 microns width and height and an aspect ratio of 1.
Although the smaller shadowed region below the high aspect ratio gridlines produced using extrusion and co-extrusion production methods provides an advantage over the low aspect ratio gridlines produced by screen printing techniques, the substantially flat upper surfaces of the rectangular shaped gridlines still reflect a significant amount of light away from the electricity generating region of the solar module. Moreover, the use of extrusion and co-extrusion production methods has also been found to introduce new problems. For example, the high aspect ratio rectangular gridlines have an undesirable tendency to fall on their side (i.e., tip over), which obviates the advantage of their high aspect ratio. In addition, the high aspect ratio extrusion process leads to stress at the silicon-silver surface that can cause delamination of the gridline. In particular, due to the different thermal expansion coefficient of silver and silicon, there is a non-negligible force on the substrate/gridline interface area that can lead to peeling of the gridlines during a cool down phase following the extrusion process. Observations indicate the peeling is generated either from breaking apart of the solar cell including a stripe of 5-10 μm thick silicon, or from insufficient etching through antireflection coating 27, which is disposed over upper surface 22 (see FIG. 18). This behavior is dependent on the peak temperature, cooling rate, on the ink, especially the glass frits content, and the ink amount.
What is needed is a micro extrusion system and associated method for forming extruded gridlines at a low cost that is acceptable to the solar cell industry and addresses the problems described above. In particular, what is needed is a micro-extrusion system for printing high aspect ratio gridlines that decreases the effective shadowed surface area of the underlying substrate, helps to prevent the tipping problem of conventional rectangular extruded gridlines, and by addressing the delamination problem associated with high aspect ratio extruded gridlines.